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Static analysis for Java Servlets and JSP
 In Proc. 13th International Static Analysis Symposium, SAS ’06, volume 4134 of LNCS
, 2006
"... Abstract. We present an approach for statically reasoning about the behavior of Web applications that are developed using Java Servlets and JSP. Specifically, we attack the problems of guaranteeing that all output is wellformed and valid XML and ensuring consistency of XHTML form fields and session ..."
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Abstract. We present an approach for statically reasoning about the behavior of Web applications that are developed using Java Servlets and JSP. Specifically, we attack the problems of guaranteeing that all output is wellformed and valid XML and ensuring consistency of XHTML form fields and session state. Our approach builds on a collection of program analysis techniques developed earlier in the JWIG and Xact projects, combined with work on balanced contextfree grammars. Together, this provides the necessary foundation concerning reasoning about output streams and application control flow. 1
The Saga of the Axiomatization of Parallel Composition ⋆
"... Abstract. This paper surveys some classic and recent results on the finite axiomatizability of bisimilarity over CCSlike languages. It focuses, in particular, on nonfinite axiomatizability results stemming from the semantic interplay between parallel composition and nondeterministic choice. The pa ..."
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Abstract. This paper surveys some classic and recent results on the finite axiomatizability of bisimilarity over CCSlike languages. It focuses, in particular, on nonfinite axiomatizability results stemming from the semantic interplay between parallel composition and nondeterministic choice. The paper also highlights the role that auxiliary operators, such as Bergstra and Klop’s left and communication merge and Hennessy’s merge operator, play in the search for a finite, equational axiomatization of parallel composition both for classic process algebras and for their realtime extensions. 1 The Problem and its History Process algebras are prototype description languages for reactive systems that arose from the pioneering work of figures like Bergstra, Hoare, Klop and Milner. Wellknown examples of such languages are ACP [18], CCS [44], CSP [40] and Meije [13]. These algebraic description languages for processes differ in the basic collection of operators that they offer for building new process descriptions from existing ones. However, since they are designed to allow for the description and analysis of systems of interacting processes, all these languages contain some form of parallel composition (also known as merge) operator allowing one to put two process terms in parallel with one another. These operators usually interleave the behaviours of their arguments, and support some form of synchronization between them. For example, Milner’s CCS offers the binary operator , whose intended semantics is described by the following classic rules in the style of Plotkin [49]. x µ → x ′ x   y µ → x ′   y y µ → y ′ x   y µ → x   y ′ x α → x ′ , y ¯α → y ′ x   y τ → x ′   y ′ (In the above rules, the symbol µ stands for an action that a process may perform, α and ¯α are two observable actions that may synchronize, and τ is a symbol denoting the result of their synchronization.)
Lifting NonFinite Axiomatizability Results to Extensions of Process Algebras
"... This paper presents a general technique for obtaining new results pertaining to the nonfinite axiomatizability of behavioural (pre)congruences over process algebras from old ones. The proposed technique is based on a variation on the classic idea of reduction mappings. In this setting, such reduct ..."
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This paper presents a general technique for obtaining new results pertaining to the nonfinite axiomatizability of behavioural (pre)congruences over process algebras from old ones. The proposed technique is based on a variation on the classic idea of reduction mappings. In this setting, such reductions are translations between languages that preserve sound (in)equations and (in)equational proofs over the source language, and reflect families of (in)equations responsible for the nonfinite axiomatizability of the target language. The proposed technique is applied to obtain a number of new nonfinite axiomatizability theorems in process algebra via reduction to Moller’s celebrated nonfinite axiomatizability result for CCS. The limitations of the reduction technique are also studied. In particular, it is shown that prebisimilarity is not finitely based over CCS with the divergent process Ω, but that this result cannot be proved by a reduction to the nonfinite axiomatizability of CCS modulo bisimilarity.
Priority and Abstraction in Process Algebra
, 2007
"... More than 15 years ago, Cleaveland and Hennessy proposed an extension of the process algebra CCS in which some actions may take priority over others. The theory was equipped with a behavioral congruence based on strong bisimulation. This article gives a full account of the challenges in, and the sol ..."
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More than 15 years ago, Cleaveland and Hennessy proposed an extension of the process algebra CCS in which some actions may take priority over others. The theory was equipped with a behavioral congruence based on strong bisimulation. This article gives a full account of the challenges in, and the solutions employed for, defining a semantic theory of observation congruence for this process algebra. A full–abstraction result is presented whose proof relies on a novel approach based on successive approximations for identifying the largest congruence contained in an intuitive but naïve equivalence. Prioritized observation congruence is also characterized equationally for the class of finite processes, while its utility for system verification is demonstrated by an illustrative example.
Complete and ready simulation semantics are not finitely based over BCCSP, even . . .
, 2011
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Is Observational Congruence on µExpressions Axiomatisable in Equational Horn Logic?
, 2007
"... It is well known that bisimulation on µexpressions cannot be finitely axiomatised in equational logic. Complete axiomatisations such as those of Milner and Bloom/Ésik necessarily involve implicational rules. However, both systems rely on features which go beyond pure equational Horn logic: either t ..."
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It is well known that bisimulation on µexpressions cannot be finitely axiomatised in equational logic. Complete axiomatisations such as those of Milner and Bloom/Ésik necessarily involve implicational rules. However, both systems rely on features which go beyond pure equational Horn logic: either the rules are impure by involving nonequational sideconditions, or they are schematically infinitary like the congruence rule which is not Horn. It is an open question whether these complications cannot be avoided in the prooftheoretically and computationally clean and powerful setting of secondorder equational Horn logic. This paper presents a positive and a negative result regarding axiomatisability of observational congruence in equational Horn logic. Firstly, we show how Milner’s impure rule system can be reworked into a pure Horn axiomatisation that is complete for guarded processes. Secondly, we prove that for unguarded processes, both Milner’s and Bloom/Ésik’s axiomatisations are incomplete without the congruence rule, and neither system has a complete extension in rank 1 equational axioms. It remains open whether there are higherrank equational axioms or Horn rules which would render Milner’s or Bloom / Ésik’s axiomatisations complete for unguarded processes.
Is Observational Congruence Axiomatisable in Equational Horn Logic?
"... It is well known that bisimulation on µexpressions cannot be finitely axiomatised in equational logic. Complete axiomatisations such as those of Milner and Bloom / Ésik necessarily involve implicational rules. However, both systems rely on features which go beyond pure equational Horn logic: either ..."
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It is well known that bisimulation on µexpressions cannot be finitely axiomatised in equational logic. Complete axiomatisations such as those of Milner and Bloom / Ésik necessarily involve implicational rules. However, both systems rely on features which go beyond pure equational Horn logic: either the rules are impure by involving nonequational sideconditions, or they are schematically infinitary like the congruence rule which is not Horn. It is an open question whether these complications cannot be avoided in the prooftheoretically and computationally clean and powerful setting of secondorder equational Horn logic. This paper presents a positive and a negative result regarding axiomatisability of observational congruence in equational Horn logic. Firstly, we show how Milner’s impure rule system can be reworked into a pure Horn axiomatisation that is complete for guarded processes. Secondly, we prove that for unguarded processes, both Milner’s and Bloom / Ésik’s axiomatisations are incomplete without the congruence rule, and neither system has a complete extension in rank 1 equational axioms. It remains open whether there are higherrank equational axioms or Horn rules which would render Milner’s or Bloom / Ésik’s axiomatisations complete for unguarded processes.
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"... Abstract This paper presents a general technique for obtaining new results pertaining to the nonfinite axiomatizability of behavioral semantics over process algebras from old ones. The proposed technique is based on a variation on the classic ideaof reduction mappings. In this setting, such reduct ..."
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Abstract This paper presents a general technique for obtaining new results pertaining to the nonfinite axiomatizability of behavioral semantics over process algebras from old ones. The proposed technique is based on a variation on the classic ideaof reduction mappings. In this setting, such reductions are translations between languages that preserve sound (in)equations and (in)equational proofs over the sourcelanguage, and reflect families of (in)equations responsible for the nonfinite axiomatizability of the target language. The proposed technique is applied to obtain a number of new nonfinite axiomatizability theorems in process algebra via reduction to Moller's celebrated nonfinite axiomatizability result for CCS. The limitations ofthe reduction technique are also studied.
Modular Semantics for Transition System Specifications with Negative Premises
"... Abstract. Transition rules with negative premises are needed in the structural operational semantics of programming and specification constructs such as priority and interrupt, as well as in timed extensions of specification languages. The wellknown prooftheoretic semantics for transition system ..."
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Abstract. Transition rules with negative premises are needed in the structural operational semantics of programming and specification constructs such as priority and interrupt, as well as in timed extensions of specification languages. The wellknown prooftheoretic semantics for transition system specifications involving such rules is based on wellsupported proofs for closed transitions. Dealing with open formulae by considering all closed instances is inherently nonmodular – proofs are not necessarily preserved by disjoint extensions of the transition system specification. Here, we conservatively extend the notion of wellsupported proof to open transition rules. We prove that the resulting semantics is modular, consistent, and closed under instantiation. Our results provide the foundations for modular notions of bisimulation such that equivalence can be proved with reference only to the relevant rules, without appealing to all existing closed instantiations of terms. 1